Method for pre-conditioning a kinetic energy storage system

ABSTRACT

A flywheel energy storage system incorporates various embodiments in design and processing to achieve a very high ratio of energy stored per unit cost. The system uses a high-strength steel rotor rotating in a vacuum envelope. The rotor has a geometry that ensures high yield strength throughout its cross-section using various low-cost quenched and tempered alloy steels. Low-cost is also achieved by forging the rotor in a single piece with integral shafts. A high energy density is achieved with adequate safety margins through a pre-conditioning treatment. The bearing and suspension system utilizes an electromagnet that off-loads the rotor allowing for the use of low-cost, conventional rolling contact bearings over an operating lifetime of several years.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.14/322,723, filed Jul. 2, 2014, which claims the benefit of and priorityto U.S. Provisional Patent Application No. 61/843,683, filed Jul. 8,2013, which is incorporated herein by reference in its entirety. Thesubject matter of this application is related to the subject matter ofU.S. patent application Ser. No. 14/322,675, filed Jul. 2, 2014, nowU.S. Pat. No. 9,735,645, and to the subject matter of U.S. patentapplication Ser. No. 14/322,749, filed Jul. 2, 2014, now U.S. Pat. No.9,136,741 issued Sep. 15, 2015.

BACKGROUND

The present disclosure relates generally to the field of kinetic energystorage. More specifically, it relates to flywheel energy storage forstationary applications where cost is of high importance; in some cases,of higher importance than weight. These applications include frequencyregulation, time-of-use, uninterruptible power supply (UPS), demandresponse, and smoothing of renewable energy generation sources, amongother applications.

Flywheels have been used as energy storage devices or for smoothingmechanical or electrical power for hundreds of years. Recently, therehave been significant advancements in the field of flywheel energystorage because of the availability of high strength-to-weight (thespecific strength) materials, like composites. The kinetic energy storedper unit mass of flywheel material can be shown to be directlyproportional to the specific strength (strength divided by density) ofthe material. Because some composite materials have very high specificstrength, composites make attractive candidates for flywheels having ahigh energy storage potential per unit mass. As an example, ahigh-strength carbon fiber composite (e.g., T700 at 70% volume fraction)has a fracture strength of 3430 mega-pascals (MPa), or 490,000 poundsper square inch (psi) and a density of 1845 kilograms per cubic meter(kg/m³), or 0.067 pounds per cubic inch (lb/in³). Compare that to anon-composite material, such as a high-strength alloy steel, which hasyield strength of 1400 MPa (200,000 psi) and density of 7870 kg/m³(0.285 lb/in³). On a strength-to-weight basis, therefore, compositeshave more than ten times higher specific strength and, therefore, areable to store more than ten times the energy per unit mass compared tosteel. This potential has led inventors to pursue designing flywheelsbased on composite rotors.

However, composite materials have not been cost-effective in stationaryapplications (i.e., applications in which weight is not the primaryconcern) where the primary goal is maximum energy stored per unit cost,rather than maximum energy stored per unit weight.

BRIEF SUMMARY

An exemplary embodiment relates to a material used for a flywheel rotorand a method used to manufacture the rotor with integral shafts. Somepreferable materials include alloy steels that are heat treatable to ahigh level of strength while maintaining sufficient ductility to enableplastic flow. Steel alloys have a high strength-to-cost ratio inaddition to low processing and fabrication costs. The rotor may beforged in multiple stages into a monolithic shape that can then bemachined to form integral shafts. Examples of suitable steel alloysinclude AISI 4340, 4330, 17PH, M300, and other high-strength alloys.

Another exemplary embodiment relates to the shape of the rotor. When asteel rotor is heat-treated, the rotors that have a higher surface areawill have a higher cooling rate. Since the cooling rate affects thematerial properties of the resulting steel, the shape of the rotor canimpact the working characteristics of the rotor. In particular, a fastcooling rate is needed to produce the transformation into martensiticsteel (a high-strength steel, desirable in flywheels). Therefore, arotor shape that allows for faster cooling may also allow for rotormaterials that have a higher proportion of martensitic steel.Specifically, a thin, disc-shaped rotor may be formable into a materialwith a higher proportion of martensitic steel than a cylinder-shapedrotor of the same volume prepared in the same way. In this situation,the disk may have a higher specific strength than the cylinder (becauseof the higher proportion of martensite) and, therefore, the disc-shapedflywheel will have a higher energy density. Since the two structureswould cost the same to make, the disc-shaped rotor would be morecost-effective because of the higher energy density. A disk may alsoexhibit a more uniform hardness (proportional to strength) throughoutthe cross-section compared to a cylinder, because the cooling rate wouldbe more uniform.

Another exemplary embodiment relates to the design of a rotor and to theuse of conventional bearings with such a rotor. It can be shown that,for a given level of stored energy, a larger diameter of flywheel rotorresults in a slower rotational speed. This slower speed allows alarge-diameter rotor to be used with conventional, low-cost rollingcontact bearings, which are highly reliable, economical, and easilymaintained, rather than non-contact systems (e.g., magnetic levitation)that must be used in designs with high rotational speeds and arecomplex, expensive, require maintenance, and compromise reliability.

Another exemplary embodiment relates to a method for reducing the loadon the bearings through the use of an off-loading electromagnet. Anelectromagnet is arranged such that it provides a vertical off-loadingforce that lifts the entire rotor against the upper bearings andpartially off of the lower bearings, reducing the load on the lowerbearings. Since bearing life is sharply reduced by increasing load, theoff-loading feature of this embodiment results in a system with a verylong bearing life compared to a non-lifted rotor system while employinglow-cost bearings and a heavy rotor. As a specific embodiment, a 5-tonrotor may be lifted by a coil of approximately 0.75 meters (30 inches)in diameter, consisting of 3420 turns of 18 AWG size copper wire.

Another embodiment relates to the use of a load cell at the upperbearing to measure the load applied to it when the rotor is lifted bythe electromagnet, and a method for using a control system that adjuststhe electromagnet's field to ensure that the desired load is alwaysapplied to the bearings. In some embodiments, this load can bemaintained at a very low value, resulting in long bearing life. Forexample, the load on the upper bearing during operation may only be 1.3kN (300 lbs) and the capacity of the upper load cell may only be 2kilo-newtons (kN) or 450 lbs for one implementation.

Another embodiment relates to the use of a control system to adjust thevoltage applied to the electromagnet to ensure that the desired load ismaintained. Load limits may be set at the controller to initiateappropriate actions should the electromagnet and/or the bearingmalfunction. A feedback loop may then be employed from the load-sensingand magnet voltage circuits to automatically maintain the correct load.

Another embodiment relates to a method in which a lower bearing is usedas a touchdown bearing that is rated to support the full weight of therotor for several hours in the event of failure of the off-loader.

In another embodiment, a load cell at the lower bearing measures theload applied to it. This is used to ensure that the desired load isapplied at start-up and that changes in loading are detected in case theelectromagnet fails during normal operation. This load cell is alsoconnected to a control system such that appropriate actions can beinitiated. In one implementation, a desired capacity of the lower loadcell for a nominal rotor mass of 5 tons is 110 kN (25,000 lbs).

Another exemplary embodiment relates to the design of the off-loadingmagnet and its low power consumption. In this embodiment, a single coilof insulated copper wire provides a suitable lifting force whilemaintaining low power loss due to the provision for a sufficiently largecross-section for the magnetic flux. In a typical application, a coil125 mm in width and 35 mm deep at an average diameter of 750 mm willprovide an off-loading force of 50 kN at a power consumption of only 45W.

Another embodiment relates to improving the reliability of the bearingsand motor/generator through the use of seals to allow for operation ofthese components in air while the rotor spins in a vacuum. Since thehigh tip speeds of the rotor will result in air drag losses, the rotoris enclosed in a vacuum housing and operated in a vacuum. Rollingcontact bearings, however, may not perform reliably for long periods ina vacuum due to outgassing of the lubricant and a tendency to formmetal-to-metal welds in a vacuum due to the lack of oxide formation aswear progresses. Also, placing the motor/generator inside a vacuum makesit difficult to cool since heat must be conducted outside of the vacuum.In such configurations, expensive heat pipes and/or large conductiveelements may need to be added to ensure adequate cooling. For liquidcooled motors, the piping carrying the coolant may need to penetrate thevacuum envelope through joints that are expensive and prone to leaking.In some embodiments, the upper and lower shafts of the rotor passthrough the vacuum envelope via low-friction fluoropolymer lip seals.This design allows for the bearings and motor to be placed outside thevacuum envelope helping to make it easy and less expensive to cool,inspect, maintain, monitor, and replace, if necessary. At the lowrotational speeds characteristic of a disk-shaped rotor, the power lossfrom the seals is small, for example, less than 50 W for 40 mm shaftseal rotating at 6000 rpm.

In another embodiment, the energy storage system is supported onseismic-rated supports to provide for lateral motion in an earthquake.Such seismic supports are used to support large buildings inearthquake-prone locations. This embodiment provides safe operation ofthe flywheel storage system if it should experience an earthquake.

Another embodiment relates to a method for increasing the energy densityof the rotor by a pre-conditioning treatment that also serves as theproof test of the rotor before it is put into service. In thepre-conditioning process, the rotor is over-spun past the yield point ofthe material. Since the maximum tensile stress occurs at the center in arotating disk of uniform thickness, yielding proceeds from the centertoward the outside diameter. If, by this process, the yield zone growsto a desired radius (for example, to about 1/√2 of the rotor radius,which corresponds to half the volume of the disk), and the disk issubsequently slowed to zero, beneficial compressive stresses at thecenter of the disk. On rotating the disk again, the resulting stressesare lower than before the over-spinning pre-conditioning process becauseof these compressive residual stresses. On reaching the rotational speedat which yielding previously occurred, the new stress levels will beless than the yield stress, helping to increase the margin of safety.This pre-conditioning process, therefore, allows one to operate the diskat a speed corresponding to the yield strength, thereby increasing theenergy density, while maintaining a positive margin of safety. Since theenergy density (the kinetic energy stored per unit mass) is proportionalto the square of the rotational speed, the increase in speed willincrease energy density stored in the rotor.

Another embodiment relates to a method in which the surface of the diskis coated with a brittle paint that indicates the stress state in therotor. The brittle paint has a very low threshold strain for brittlefracture and serves as an indicator of the magnitude of the stresses inthe rotor and its distribution. As the rotor increases in speed, thestrain corresponding to the rotor's stress state is recorded in thecoating through a pattern of fine cracks. The spacing between the cracksis a measure of the stress; cracks closer together signify a higherstate of stress than cracks further apart. By loading a tensile sampleof the same material with the same coating, the crack spacing can becalibrated with respect to the stress. This technique helps one toestimate not only the magnitude and direction of the stressesexperienced by the rotor, but also the stress distribution. Theseestimated values can be compared with analytical results to verify thefidelity of a computational model used in analysis of the rotor. Inaddition, the stress distribution obtained in this manner corresponds toeach specific rotor that is tested. Thus, accurate statistics can beobtained on the manufacturing variability between rotors, helping toprovide a quantitative measure of the reproducibility and reliability ofthe manufacturing process that was used to form the disk.

In another embodiment, an arrangement is described in which a videocamera and a strobe light placed inside the vacuum envelope allows forreal-time observation of the stress state in the rotor. The frequency ofthe strobe light is synchronized with the rotational speed of the rotor,facilitating real-time observation of the progression of the cracks inthe brittle paint and, therefore, the stress distribution in the rotor.This capability may be useful for determining the relative margins ofsafety during operation of the system, particularly during thepre-conditioning process when an accurate measurement of the progressionof the plastic zone with speed is essential.

Another embodiment relates to a method in which strain gages coupledwith transmitters and receivers are used to monitor the stress state inthe rotor. In this embodiment, strain gages are bonded to the surface ofthe rotor at locations of interest parallel and tangential to the radiusvector. The addition of a telemetry transmitter to each strain gageallows one to read the strain in real time as the rotor rotates. Areceiver inside the vacuum envelope and attached to the housing receivesthe strain gage reading and transmits it via a cable connected to acomputer for display and recording. This arrangement provides real-timemeasurement of the strain distribution in the rotor while it isrotating, information that may be particularly important during thepre-conditioning process since the stress distribution and the extent ofthe plastic zone can be accurately tracked with rotor speed.

Another embodiment relates to a method for reducing theprecession-induced moment on a spinning rotor arising from the earth'srotation, while maintaining a high resonant frequency in the rotor/shaftarrangement. Thrust bearings alone are not adequate to absorb thismoment at high rotational speeds. In an exemplary embodiment, theprecession-induced moment on a spinning rotor arising from the earth'srotation is resisted by two angular contact bearings at the ends of therotor shafts. The angular contact bearings provide axial support duringoperation and radial (or lateral) loading capability to resistprecession-induced loads.

Another embodiment relates to an integrated vacuum housing and supportstructure for a flywheel rotor. Such an integrated housing may help tominimize the number of parts and reduce cost. The housing may bedesigned to maintain a vacuum in the space occupied by the rotor. Tominimize cost and number of components, the vacuum chamber also servesas: the structure that supports the rotor; the alignment fixture for theshaft and bearings; and a suspension system for the rotor. The top plateof the vacuum envelope may also serve as a suspension element. Duringoperation, the rotor is lifted by the electromagnet, which is integratedstructurally into the top plate of the vacuum chamber. The stiffness ofthe top plate is designed so that, when the rotor is suspended, theminimum resonant frequency of the systems is at a value that is wellbelow the operating speed range of the rotor. This arrangement helps toprevent fundamental resonances from occurring during normal operation ofthe system.

Another embodiment provides a method for adjusting the stiffness of thetop plate by adding or removing radial rib stiffeners, thereby providinga means for promoting resonances at the desired rotational speeds.

Another embodiment is a low-cost way for an accurately aligned systemwith tailored stiffness using three components: an upper plate, a lowerplate, and a cylindrical section. By manufacturing the upper and lowerplates from cast iron and the cylindrical section from a standard pipesection, one obtains an economical yet strong design. Ribs or stiffenerscan be added or removed by welding to, or machining from, a basic castiron form.

Another embodiment is a method for the use of dowel pins to accuratelydetermine the relative position of the upper and lower plates of thethree-component system.

Another embodiment is a method for the use of recessed lips in the upperand lower plates that seat at the ends of the cylindrical section tolocate the upper and lower plates accurately with respect to each other.

In another embodiment, integral O-ring seal grooves at the two ends ofthe cylindrical section provide a low-cost mechanism for ensuring aleak-proof removable joint in the vacuum system.

In another embodiment, the electromagnet structure is integrated intothe upper plate of the three-component system, resulting in anintegrated structure that is multi-functional, being operable as both ahousing for the coil, and a cross-section for the magnetic flux that islarge enough to preclude saturation.

In another embodiment, a bearing/seal pack at each of the two bearinglocations provides a convenient means to remove and inspect bearingswithout disassembling the system.

In another embodiment, the upper bearing pack has a means for accuratelylocating the axial position of the rotor shaft with respect to the airgap between the electromagnet and the rotor.

In another embodiment, the lower bearing pack has a means for accuratelylocating the axial position of the rotor with respect to the air gapbetween it and the electromagnet. With this embodiment, relativedisplacements between the upper and lower bearings resulting fromdeflections in the top and bottom plates due to rotor weight and/orvacuum pressure are compensated for such that there is adequate axialclearance between the bottom shaft stop and the lower bearing duringoperation.

In another embodiment, compact low-profile wavy springs ensurepreloading the bearings in each bearing pack. A minimum axial preload isnecessary to prevent ball-to-race sliding (instead of rolling) at highspeeds that causes the temperature to rise, which in turn, can result inlubricant break-down leading to bearing failure.

In another embodiment, an actuator, such as a motor-driven worm gear, atthe base of the unit provides a means for adjusting the axial positionof the rotor remotely and autonomously when used in conjunction with adisplacement transducer and a control system.

Another embodiment provides a means for lifting the rotor after initialassembly so that it is at the desired air gap to be magnetically held bythe electromagnet. The application of vacuum to the inside of the sealedhousing results in downward displacement of the top plate, and upwarddisplacement of the bottom plate, due to the external atmosphericpressure. When the rotor is stationary and is resting on the lowerplate, the force due to the external atmospheric pressure is sufficientto lift the rotor by deflecting the bottom plate such that the rotorshaft contacts the bearing stop at the upper bearing pack. This featureprovides a means for achieving the desired air gap between the rotor andthe magnet so that an adequate force to lift the rotor can be achieved.For example, a housing of 1.85 m (73 inches) in diameter will result ina force of 271 kN (61,600 lbs) applied downward on the top plate and thesame force applied upward to the bottom plate. For a 5-ton rotor,resting on the bottom plate, this force is sufficient to lift the rotorat a pressure differential of about 20% of sea-level atmosphericpressure. By adjusting the level of vacuum, the amount of liftdisplacement of the rotor can be controlled. This embodiment is alow-cost yet effective means for positioning the rotor to enable it tobe magnetically held prior to rotation.

Another embodiment describes a method for supporting the rotor. A hollowcylindrical structure located on the axis and at the bottom of the lowerbearing pack acts as a single adjustable foot that supports the weightof the rotor when the off-loader is not activated.

Another embodiment describes the arrangement of several adjustable feetlocated below the bottom plate and under the cylindrical pipe section ofthe housing.

Another embodiment describes a method for seismic isolation of thesystem by adding discrete isolators at each foot.

Another embodiment describes a method for seismic isolation through theuse of a continuous flexible support such as a thick rubber sheet placedunder the bottom plate that allows for sliding as well as shear.

Another embodiment describes the use of non-contacting displacementsensors, such as capacitive gages, located on the inside of the vacuumchamber and spaced around the periphery of the rotor that measures thechange in radius of the rotor with speed.

Another embodiment describes a means for determining and removingdynamic imbalances in the rotor. Accelerometers are mounted around theperiphery of the bearing packs to measure the level of imbalance. Theaccelerometer signals are correlated with the motor rotary encoder forprecisely determining the angular location of the net imbalance in therotor. This information is used to remove a small amount of material atthe periphery corresponding to the imbalance location to reduce orremove the imbalance.

Another embodiment describes the use of displacement gages to measurethe axial displacement of the rotor relative to the structure.Displacement gages, such as extensometers, are mounted at the base ofthe unit within the bearing pack to record the dynamic (axial) motion ofthe suspended rotor over its entire operating and pre-conditioning speedranges to determine the speeds at which the rotor experiences eachresonant mode. This embodiment provides the axial component of thedisplacement alone, which is valuable since one is able to characterizethe axial component of the dynamic response of the suspended rotor overvarious operational modes.

In another embodiment, temperature sensors are placed adjacent to or onthe outer races of the upper and lower bearings to monitor temperaturechanges that may signal potential failure and/or wear.

In another embodiment, a torque noise sensor is placed beneath the upperand/or lower bearing. The signal from this sensor, when compared withthe signal from the torque transducer at the motor-to-rotor coupling, isa measure of wear in the bearing and provides for early detection of apotential bearing failure.

In another embodiment, acoustic emission (AE) sensors are placed on thestructure at several locations including at the bearing packs and insidethe vacuum housing. The transducers are in close contact with thestructural elements via gel or grease acoustic coupling media.

In another embodiment, individual or bundles of ultra-high modulus (UHM)carbon fiber are bonded (or otherwise attached) tangentially andradially to the rotor surface at various radial distances from the rotoraxis. Since the failure strain of UHM carbon fiber is low (˜1000×10⁻⁶)relative to strains experienced by the rotor when spinning (˜5000×10⁻⁶),the individual fibers will begin to fail as the strain in the rotorincreases with speed. Fiber failures have a characteristic AE signature,which can be detected by an AE sensor (for example, a 500 kHz sensor)bonded to the structure near the rotor bearing location. This embodimentprovides a means to determine the strains in the rotor remotely whilewithin the vacuum envelope. The method can be used in other applicationswhere strain gages or other methods cannot be used, for example, inhostile environments, such as high temperature and/or oxidative andcorrosive atmospheres. Other fibers, such as mineral, glass and polymerfibers, may also be similarly employed for different levels of failurestrain capacity.

Another embodiment describes a means for efficient energy absorption inthe event of rotor burst failure. A buried thick-walled steel andconcrete containment structure is constructed in close proximity, andpreferably, in contact with the outside cylinder wall of the housing.This arrangement keeps fragments from rotor failure to be containedwhile still in rotational modes (minimizing translational modes) so thatenergy dissipation is facilitated by friction and particle-to-particleinteraction.

In another embodiment, the containment structure is constructed with atapered geometry such that the diameter of the containment structureincreases gradually with increasing depth from the bottom of the unit.At rotor failure, the fragments will tend to displace axially downwardand be collected below the unit rather than move upward and be ejectedabove the surface.

In another embodiment, an arrangement of graded aggregate is placed suchthat aggregate size decreases with radial distance from the concretewall. This results in an energy-absorbing structure with larger porosityadjacent to the concrete containment structure where, crushing andcompaction of the aggregate provides energy absorption. At increasingradial distance, the decreasing size of the aggregate approaches that ofsand particles that are also arranged with decreasing particle size withincreasing radial distance. In this zone, fragment motion is resisted byfriction with the sand particles.

In another embodiment, bearing packs, each including an accuratelyaligned bearing/seal/load cell assembly, are contained in housings thatare provided with dowel pins or locating features that accurately locatethe axis of each with respect to the housing axis and, therefore, witheach other.

In another embodiment, control software provides for safe operation ofthe system over its various modes of operation: pre-conditioning, speedcycling, power cycling, demand response, time-of-use, and otherstrategies for maximizing the benefits of storage with respect to thegrid and/or other generating sources such as renewables (solar, wind,tidal) and/or diesel or gas-powered generators.

In another embodiment, control logic is incorporated in the controlsoftware for safe and efficient operation under various potentialfailure scenarios including, but not limited to, failures of themotor/generator, bearings, off-loader, vacuum pump, cooling systems,seismic events, and temperature spikes.

In another embodiment, the rotor is connected to an electronic ormechanically controlled continuously variable transmission (CVT) orother geared transmission such that the varying speed of the rotor isoutput to an induction motor. Over-driving the induction motor in thisfashion past the slip speed results in power output, while under-drivingit will result in the induction motor being driven by the external powersource to store kinetic energy in the rotor by increasing its speed toits maximum rated value. This is a low-cost method for energy storageand delivery since it does not involve brushless DC motors and theirassociated control and driver software schemes.

In another embodiment, a radial temperature gradient is maintained alongthe rotor radius. When the center of the rotor is at a highertemperature than its periphery, a non-uniform thermal strain is createdthat results in a beneficial thermal stress (compressive at the center,tensile at the periphery), which improves the overall stress state andthereby increases the energy density in the rotor.

In another embodiment, the geometry of the rotor is a simple fixed orvariable thickness disk without shafts. Shafts are machined separatelyfrom alloy steel that may be austenitic (and, therefore, non-magnetic)and bonded to the disk. Since the rotor is lifted directly by themagnetic off-loader, the stresses in the bond joints are low andprimarily compressive, due to the axial compressive preload, and areeasily accommodated by the bond strengths of conventional polymerstructural adhesives.

In another embodiment, the rotor is a simple fixed or variable thicknessdisk without shafts. Shafts are machined separately from alloy steelthat may be austenitic (and, therefore, non-magnetic) and welded to thedisk. Following the welding operation, conventional heat treatmentprocedures remove stress concentrations introduced into the rotor at theweld locations. The magnetic off-loader lifts the rotor directly and notby its shafts; thus, the stresses in the welds are low.

In another embodiment, the rotor is made as a simple fixed or variablethickness disk without shafts. Shafts are machined separately from alloysteel that may be austenitic (and, therefore, non-magnetic). The shaftsare friction-welded to the disk by spinning them up to a high speed andthen axially pressing them onto the disk. Following the friction-weldingoperation, conventional heat treatment procedures remove any stressconcentrations introduced into the rotor at the friction-welds. Sincethe rotor mass is lifted by the magnetic off-loader, the stresses in thewelds are low.

In another embodiment, the rotor includes several laminated plates thatare adhesively bonded together using conventional structural adhesives.The only stress in the joints between the laminations is gravityloading, which occurs when the rotor is lifted. This stress is low andeasily accommodated by the adhesive tensile strength. For example, forten laminations, each 25 mm in thickness (1 inch), the tensile stress inthe first lamination joint (the most highly loaded bonded joint) is lessthan 0.021 MPa (3 psi). Structural adhesives have tensile strengthsreadily exceeding 7 MPa (1000 psi). Thin laminas can be individuallyheat-treated to higher strengths, thereby increasing the rotor energydensity. In addition, laminated rotors have a high degree of redundancysince flaw propagation in one lamina tends to be restricted by theadjacent laminas. In addition, failure of one lamina does not result infailure of the entire rotor. Also, since the laminas are thin, they arein a state of biaxial plane stress when the rotor is spinning, a moreuniform stress state corresponding to a higher energy density, than thebiaxial plane strain state that exists in a thick monolithic rotor.

In another embodiment, the materials used in each lamination may bedifferent for a fail-safe failure mode. For example, the laminationsadjacent to the shafts may be made from a ductile yet relatively lowerstrength steel since fracture of the shaft-to-rotor failure would becatastrophic. The inner laminations may be made from a higher strengthsteel whose failure would be detectable and would not be catastrophic.

Another embodiment describes the use of permanent magnets instead ofelectromagnets. This arrangement is a more reliable, less expensive, andless complex off-loading scheme, since the power supply, coil, leads andfeed-through connections are not required.

In another embodiment, a remotely controlled actuator establishes anadjustable and controllable air-gap between the rotor and the permanentmagnet off-loader.

In another embodiment, the air-gap between the rotor and theelectromagnetic or permanent magnet off-loader is maintained throughfeedback from the load cell that measures the lifting magnet forces.This arrangement provides closed loop control of lift loads that mayvary due to dynamics, wear, and temperature variations during operation.

In another embodiment, single or multiple coupled DC motor/generatorspowered by DC power from two inverters mounted on the downstream end ofa bidirectional controller connected to the grid at 460V, 3 phase (orother distribution voltages) is a low-cost scheme for energy storage atgrid-scale. The arrangement provides modularity in both energy storageand power. For example, a 150 kWh capacity flywheel coupled with a 30 kWmotor/generator can deliver 30 kW continuously for 5 hours to takeadvantage of differential pricing for time-of-use storage. For demandresponse and higher power, short-time applications, a motor/generator of150 kW rating can be readily substituted to deliver 150 kW for 1 hour.The addition of a second 150 kW motor/generator at the bottom shaftlocation doubles the power rating by supplying 300 kW power for 30minutes.

Another embodiment relates to a method for high-speed manufacture ofcomposite rotors. In this embodiment, a composite fiber-reinforced ringis manufactured using a high-speed rotating cylindrical mold into whichis fed a fiber bundle from a rotating spool located inside the mold. Thespin axis of the fiber-dispensing spool is parallel to the rotating moldaxis. As the fiber bundle is unwound from the spool, it is held againstthe inside surface of the rotating mold by centrifugal force. Roomtemperature curing pre-catalyzed thermosetting resin is sprayed from anozzle perpendicular to the vertical wall of the rotating mold onto thefiber bundle lying against the wall. The high g-force provides adequatepressure for the liquid resin to infiltrate the fiber bundle as curingof the resin proceeds. When the cure is complete, the mold is stoppedand the ring ejected from the mold. This process is 10 to 50 timesfaster than filament winding, the conventional process for manufacturingcomposite rings. For example, fiber dispensing rates of 4,500 m/min arepossible with this arrangement compared to filament winding rates of60-90 m/min. Alternatively, a resin system that cures at elevatedtemperature may be used, together with a method for heating the moldsurface either by internal electrical resistance heaters, gas firedheaters, or infrared lamps illuminating the mold from the inside.Alternatively, the rotating mold has a central shaft and shaft lip sealsso that infiltration and curing is done in vacuum to minimize voids inthe composite. Additional spools may be simultaneously deployed suchthat processing times can be further reduced and/or different fibers orwires (glass, carbon, Kevlar, polymers, metal wires, etc.) can bedispensed simultaneously or in sequence such that the final compositering has a layered or mixed configuration of different fiber types,which may be advantageous for certain applications. Alternatively,different resin systems can be applied in sequence to vary propertiesradially. For example, a composite ring can be readily fabricated inthis manner with carbon fibers at its outside diameter and glass fibersat its inside diameter. Due to the high g-loading in this embodiment,for example, 300 g's in a 2 m diameter mold rotating at 520 rpm,void-free composite rings can be produced at high rates.

In another embodiment, a metal wire coil is manufactured using ahigh-speed rotating cylindrical mold into which is fed a metal wire,such as copper wire, dispensed from a rotating spool located inside themold whose spin axis is parallel to the rotating mold axis. As the fiberbundle is unwound from the spool, it is held against the inside surfaceof the rotating mold by centrifugal force. Room temperature curingpre-catalyzed potting resin is sprayed from a nozzle perpendicular tothe vertical wall to pot the coil for use as an electromagnet coil, forelectric motors, or other electrical devices.

Another embodiment describes a method for making a composite ring usinga pre-impregnated fiber bundle, or tow preg, that is dispensed into ahigh-speed rotating cylindrical mold, a rotating spool located insidethe mold whose spin axis is parallel to the rotating mold axis. As thefiber bundle is unwound from the spool, it is held against the insidesurface of the rotating mold by centrifugal force. Infrared, hot air, orother types of heaters provide the heat for curing the matrix polymer inthe tow preg. As before, various fibers and/or metal wires can bedispensed in this manner, simultaneously or sequentially.

BRIEF DESCRIPTION OF THE DRAWINGS

The exemplary embodiments will be explained in more detail in thefollowing text with reference to the attached drawings.

FIG. 1 is a process flow drawing showing a sequence of processing stepsfor the manufacture of a high-energy density rotor at low cost.

FIG. 2 is a schematic drawing showing a kinetic energy storage device inthe form of a spinning rotor supported by bearings inside a vacuumenvelope and driven by an external motor/generator.

FIGS. 3A-3D show plots of the beneficial effect of pre-conditioning onthe stress state in the rotor.

FIG. 4A is a schematic drawing showing the use of a brittle paintcoating for determining the stress state in the rotor.

FIG. 4B is an example pattern of cracks in the brittle paint coating,the cracks resulting from spin up of the rotor.

FIG. 5 is a schematic drawing showing an arrangement of a video cameracoupled with a strobe light for obtaining images of the crack patternsin the brittle paint coating while the rotor is spinning

FIG. 6 is a schematic drawing showing an arrangement of strain gagesconnected to radio frequency transmission circuits and antennas fordetermining the strains in the rotor while it is spinning.

FIG. 7 is a schematic drawing showing us of the top plate of the vacuumhousing as an elastic suspension element of the rotor.

FIG. 8 is a schematic drawing showing stiffening ribs in a top plate,which acts as a suspension element of the rotor, the added or removedribs altering the stiffness of the top plate and, therefore, theresonant frequency of the rotor suspension system.

FIG. 9A is a schematic drawing showing how the top plate (and upperbearing) may be accurately located with respect to the bottom plate (andlower bearing) through a machined recessed lip for precise alignment ofthe rotor axis.

FIG. 9B is a schematic drawing showing how the lower central support maybe raised or lowered to maintain the desired air gap between the rotorand the lifting off-loading magnet by employing a motor-driven mechanismsupported on thrust bearings.

FIG. 10 is a schematic drawing showing details of the central supportfoot.

FIG. 11 is a schematic drawing showing the use of a rubber orelastomer-based sheet for providing seismic isolation to the unit.

FIG. 12 is a schematic drawing showing the use of displacement gages formonitoring rotor diameter change while spinning.

FIG. 13 is a schematic drawing showing the use of accelerometers tomeasure and monitor rotating imbalances in the rotor.

FIG. 14 is a schematic drawing showing the use of an extensometer tomeasure axial shaft displacement and vibration during operation.

FIG. 15 is a schematic drawing showing the use of acoustic emissionsensors for monitoring bearing wear and progressive damage in the deviceduring operation.

FIG. 16 is a schematic drawing showing a containment design forcapturing fragments from a failed rotor.

FIG. 17 is a schematic drawing showing a graded aggregate and sandarrangement for stopping fragments released during a rotor failure.

FIG. 18 is a schematic drawing showing an arrangement for using aninduction motor as a motor/generator when coupled to the rotor through acontinuously variable transmission (CVT).

FIG. 19 is a schematic drawing showing the imposition of a thermalgradient in the rotor to improve the storage energy density through theintroduction of beneficial thermal stresses.

FIG. 20 is a schematic drawing showing a method for the attachment of aseparately machined shaft to a rotor by adhesive bonding.

FIG. 21 is a schematic drawing showing a method for the attachment of aseparately machined shaft to a rotor by fusion or friction welding.

FIG. 22 is a schematic drawing showing a rotor made from severallaminations.

FIG. 23 is a schematic drawing showing a method for the rapidmanufacture of a composite ring using dry fiber bundles dispensed into arotating mold, together with pre-catalyzed resin.

FIG. 24 is a schematic drawing showing a method for the rapidmanufacture of a composite ring using pre-impregnated fiber bundles (towpreg) dispensed into an internally heated rotating mold.

DETAILED DESCRIPTION

With reference to the accompanying figures, the present disclosurerelates to kinetic energy storage, specifically flywheel-based energystorage, for use in electrical grids, renewable energy generationsystems such as wind turbines, solar panels, tidal machines, andindustrial applications where smoothing of power demand reduces bothcapital and operational costs. The present disclosure also relates tomethods of producing, controlling, and integrating such storage deviceswith existing grid and micro-grid energy distribution systems. While thesubject matter herein is presented in the context of energy storagedevices in the field of grid-scale applications, such devices may beutilized in alternate applications such as stand-alone energy storagefor electric vehicle charging stations, rail transportation systems,elevators, cranes, shipboard systems, or any other devices utilizingkinetic energy storage, as will be appreciated by those of skill in theart who review this disclosure.

Referring to FIG. 1, an exemplary sequence of metal-forming operationsis shown for producing a rotor with the desired strength and uniformityat low cost. The rotor may be one of the most expensive components inthe design of the energy storage device disclosed herein. It may be ofconstant or variable thickness. When rotating at high speed, thestresses in a constant thickness rotor are at a maximum at its centerwhere the radial and tangential stresses are both tensile. Structuralintegrity at the center is, therefore, more important than materialintegrity at the edges, since flaws are more likely to initiate andpropagate at the center of the rotor. The manufacturing sequence shownin FIG. 1 is a method for helping to reduce the size of the flaws in aneconomical and reproducible manner.

A cast ingot of the desired alloy, for example, American Iron and SteelInstitute (AISI) designation 4340, is cut to the desired volume andsubjected to one or more upsetting operations in an open die set-up in ahydraulic press at the hot forging temperature. This process compressesvoids in the ingot and stretches inclusions into thin and finerparticles called stringers. Since the loading is axisymmetric, theprocess may also result in dispersion of stringers. In an exemplaryembodiment, the blank is further hot-forged into a shape containingbosses on either surface using a closed-die set of tools. In someembodiments, the heights of the bosses exceed the final heights of theintegral shafts of the rotor. The bosses may be of different heights forspecific applications. Following this operation, the rotor is now almostin its final shape. This shape may present a relatively thincross-section for rapid and uniform cooling during the quench operationin the heat treatment process.

Transformation-hardening steel alloys such as AISI 4340 depend upon aminimum cooling rate for the formation of martensite, which, after thetempering process, determines the strength and ductility of the finalproduct. The minimum cooling rate occurs in the thickest location of thecross-section farthest from the surface. Thus, the design of the rotor,for maximum energy storage density capacity at minimum cost, dependsupon a low aspect ratio (thickness-to-diameter ratio). In one example,an aspect ratio of about 15% results in a thickness of 0.25 m (10inches) for a maximum energy storage capacity of 150 kWh when AISI 4340heat-treated alloy steel is used. In other embodiments, thicknesses ofless than 0.25 m may be used (e.g., thickness in the range of 0.05m-0.25 m).

Following the closed-die operation to form the bosses, the blank isrough-machined to further reduce the maximum thickness in the blank.This process may be followed by quenching and tempering (heat-treating).An exemplary quenching is to heat the blank to 850° C., quench in apolymer-modified water bath, followed by tempering at 210° C.-250° C.Following the quenching operation, the part is finish-machined andbalanced. Such a process sequence may result in a minimum yield strengthof about 1200 MPa (170,000 psi), ultimate tensile strength of about 1300MPa (185,000 psi), and ductility of at least 6% for an exemplary rotorof the dimensions discussed above. It may be important to ensureadequate ductility so that the rotor, when subjected to thepre-conditioning process disclosed below, will have the desiredbeneficial residual stress state that improves energy density andensures adequate margins of safety.

Referring to FIG. 2, a system 10 shows a flywheel energy storage devicethat includes a rotor 12 that is located within a hermetically sealedhousing including a top plate 14, a cylindrical vertical enclosure 16,and a bottom plate 18. Two bearing packs 20 allow the rotor to rotatefreely in rolling contact with the bearings held within each bearingpack. Dowel pins 22 accurately locate the upper and lower plates withrespect to each other. O-ring seals 24 in the cylindrical enclosure 16seal the top and bottom plates to form the vacuum enclosure. Ribs 26 inthe top and bottom plates provide the desired level of stiffness to eachplate. An electromagnet 28 in close proximity to the top surface of therotor provides a vertical force large enough to lift the rotor. Anannular slot 30, whose axis coincides with the axis of the rotor, is cutinto the body of the electromagnet. The annular slot is filled with acopper coil 32 including several coils of a single insulated wire that,when connected to a DC power supply, will provide a controllable liftingforce on the rotor.

A series of externally mounted feet 34 support the device on a pad 36including a number of bonded and laminated steel/rubber layers thatprovide isolation to the device from seismic events. The bearing pack 20contains a lip seal 38 that seals the rotating shaft against airinfiltrating into the vacuum envelope. A wavy spring 40 ensures that aminimum axial preload exists on the rolling contact bearing 42 and aload cell 44 provides a means for tracking the axial load on the bearingduring operation. The shaft of the rotor 12 has a series of stepsmachined into it to accommodate the seal, spring, bearing, and loadcell. The bearing pack outer housing 46 is located accurately on the topplate via dowels 48. The axial position of the shaft is adjusted by aninternally threaded hollow cylindrical insert 50 that, when rotated,establishes the upper set point that locates the load cell's (and,therefore, the shaft's) axial position. This feature provides a meansfor adjustment of the air gap between the top surface of the rotor 12and the electromagnet 28. A coupling shaft 52 connects the top of therotor shaft to the motor/generator 54.

FIGS. 3A-3D show plots of the stress distribution in the rotor when apre-conditioning treatment as disclosed below is performed on the rotor.FIG. 3A shows the stress distribution (radial and tangential stresses)in a rotor spinning at a speed at which yielding just begins to occur atthe center of the rotor. This point is considered to be the maximumlevel of loading for the rotor and its maximum operating speed isusually set to a value well below this value. However, increasing therotor speed above the point corresponding to the initiation of yieldcreates a plastic zone that grows as shown in FIG. 3B to a radius r_(p).On reducing the rotor speed to zero, a residual stress state now existsas shown in FIG. 3C, which is characterized by a central compressivezone. On re-spinning the rotor to the speed reached in FIG. 3A, theresidual compressive zone reduces the maximum stress so that a positivemargin now exists at the speed corresponding to the yield speed. Thispre-conditioning process thus increases the energy storage density inthe rotor.

In some embodiments, the rotor strain may be estimated usingcomputational models. In such an embodiment, the desired amount ofstrain may be converted to the rotation speed for a given rotor materialand geometry. In this way, a sufficient amount of strain would simply bea given spin speed, without actually measuring the strain in each rotor.In other cases, as will be shown, the strain may be measured whilespinning is carried out, such that the strain may be determined and thespinning speed may be increased until the desired yielded zone isproduced.

FIG. 4A shows the application of a brittle paint 56 onto the rotor 12.On spinning up the rotor, the strain in the rotor produces a crackpattern 57, shown in FIG. 4B, in the brittle paint that represents thestress state in the rotor. The crack pattern includes tangential andradially distributed cracks whose spacing is a measure of the magnitudeof the stress; the closer the spacing, the larger the stress.Quantitative values of the stress distribution can be obtained throughcalibration from loading a tensile specimen to known loads and measuringthe crack pattern. In addition to the magnitude of the stresses, thedirections of the principal stresses are also displayed in the patternsince the orientations of the cracks are perpendicular to the principalstress directions.

FIG. 5 illustrates the use of a video camera 58 and a strobe light 60whose frequency is synchronized with the rotor speed. In this manner,the progression of the cracks in the brittle paint layer on the rotor 12within the vacuum envelope of vertical enclosure 16 can be recorded as afunction of rotor speed.

In FIG. 6, strain gages with radio-frequency (RF) transmitters 62 arebonded to the surface of the rotor 12 inside the vacuum chamber wall 16and oriented along directions of interest parallel and tangential to theradial vector. A receiver inside the vacuum envelope communicates thestrain gage readings to a recorder via a cable for display andrecording. This arrangement provides real-time measurement of the straindistribution on the rotor while it is rotating, information that isparticularly important during the pre-conditioning process, since thestress distribution and the extent of the plastic zone is accuratelytracked with rotor speed. In addition, control software can use thisinformation to warn of responses that are not nominal, and shut down theunit, if necessary.

FIG. 7 is a sketch that illustrates the use of the elastic response ofthe top plate from which the rotor is suspended as a spring thatdetermines the minimum resonant frequency of the system. The weight ofthe rotor deflects the top plate depending upon its stiffness. Theresonant frequency is proportional to the square root of the ratio ofthe plate stiffness (the rotor weight divided by the deflection of theplate, shown as the dotted line in the figure) to the rotor weight.Thus, if the stiffness of the top plate can be adjusted, one can obtaina desired resonant frequency of the system. This feature is illustratedin FIG. 8, which shows how the stiffness of the top plate 14 can beadjusted by adding or removing rib stiffeners 64. The lateral loadsdepend, for a given rotor speed, on the lengths of the rotor shafts,with the load decreasing with increasing shaft length. The resonantfrequency of the first-bending mode of the rotor/shaft structure,however, increases with decreasing shaft length. While the resonantfrequency decreases with shaft length as L^(3/2), it increases withshaft diameter as d². Thus, a suitable ratio of the shaft diameter tolength provides a system that has both low lateral loading on thebearings from rotor precession as well as high resonant frequency.

Referring to FIG. 9A, a recessed lip 66 in the top plate 14 accuratelylocates it with respect to the vacuum chamber wall 16. This feature,also present in the bottom plate, ensures that the alignment between thetop and bottom bearing packs is accurate.

Referring to FIG. 9B, a worm gear 68 is used to accurately locate theaxial position of the bearing pack 20 and, therefore, the rotor withrespect to the air gap between it and the electromagnet. The worm gearis driven by a motor (not shown), or manually, to rotate the outputshaft 70, which, by virtue of a screw mating with the bearing pack,lifts or lowers the entire assembly. With this embodiment, relativedisplacements between the upper and lower bearings due to deflections inthe top and bottom plates resulting from rotor weight and/or vacuumpressure are compensated for, such that there is adequate axialclearance between the bottom shaft stop and the lower bearing duringoperation. These adjustments can be carried out remotely and, ifnecessary, autonomously when used in conjunction with a displacementtransducer and controller.

Referring to FIG. 10, a hollow cylindrical structure 72 located on theaxis and at the bottom of the lower bearing pack acts as a singleadjustable foot that supports the bottom plate 18 when the rotor 12 isstationary and/or the off-loader is not activated.

Referring to FIG. 11, the entire unit is placed on a thick rubber sheet,or a laminated assembly of steel plates and rubber sheets 74 to provideseismic isolation.

Referring to FIG. 12, non-contacting displacement sensors 76, such ascapacitive gages, located on the inside of the vacuum chamber wall 16and spaced around the periphery of the rotor 12 determines the change inradius of the rotor with change in its speed. This information is usefulto verify the numerical model as well as warn of anomalous displacementchanges that may indicate impending rotor or bearing failure.

Referring to FIG. 13, two or more accelerometers 78 are mounted aroundthe periphery of each bearing pack to measure the level of imbalance.The amplitudes of the accelerometer signals provide information on themass of the imbalance when the rotor speed is known. When the timesignature of each accelerometer signal is correlated with the motorrotary encoder, the angular location of the net imbalance in the rotorcan be identified and removed in a subsequent machining operation. Inaddition, changes in the accelerometer signals during operation can beused as indicators of bearing wear and/or impending failure of thesystem.

Referring to FIG. 14, a displacement gage 80 is mounted at the base ofthe unit within the bearing pack to record the dynamic (axial) motion ofthe suspended rotor over its entire operating and pre-conditioning speedranges to determine the speeds at which the rotor experiences eachresonant mode. This information can also be used to indicate anomalousbehavior of the system.

Referring to FIG. 15, acoustic emission (AE) sensors 82 are placed onthe structure at several locations, including at the bearing packs andinside the vacuum housing. These sensors measure high-frequency (forexample, 500 kHz) sounds emanating from bearings and or flaw propagationin the rotor, thereby providing a measure of the wear or impendingfailure of one or more components in the system.

Referring to FIG. 16, a buried thick-walled steel and concretecontainment structure 84, 86 is constructed to be in close proximity,preferably, in contact with the outside cylinder wall of the device 10.This arrangement keeps fragments resulting from rotor failure to becontained in rotational modes (minimizing translational modes) so thatenergy dissipation is facilitated by friction and particle-to-particleinteraction. The containment structure 84 has a tapered geometry, suchthat the diameter of the containment structure increases gradually withincreasing depth from the bottom of the unit. At rotor failure, thefragments will tend to displace axially and be collected below the unitrather than move upward and be ejected above the surface.

Referring to FIG. 17, an arrangement of graded aggregate 88 is placedsuch that aggregate size decreases with radial distance from theconcrete wall. This results in an energy-absorbing structure with largerporosity adjacent to the concrete containment structure and decreasingsize of the particles with increasing radial distance.

Referring to FIG. 18, the device 10 is connected to an induction motor90 through an electronic or mechanically controlled continuouslyvariable transmission (CVT) 100 or other geared transmission.Over-driving the induction motor in this fashion past the slip speedmakes it operate like a generator outputting power to the grid.Under-driving the motor by changing the gear ratio in the CVT willresult in the induction motor being driven by the external power sourceto accelerate the rotor and thereby store energy. This is a low-costmethod since it does not involve brushless DC motors, inverters, andtheir associated control and driver software schemes.

Referring to FIG. 19, a radial temperature gradient is imposed on therotor 12 by heaters 110. When the center of the rotor is at a highertemperature than its periphery, the resulting non-uniform thermal strainresults in beneficial thermal stress (compressive at the center, tensileat the periphery), which improves the overall stress state and therebyincreases the energy density in the rotor.

FIG. 20 illustrates a concept for using discrete, separately machinedshafts 120, which may be made from an alloy steel that may be austenitic(and, therefore, non-magnetic) and adhesively bonded to the rotor 12with a structural adhesive 122. Since the rotor is lifted directly bythe magnetic off-loader, the stresses in the bond joints are low andprimarily compressive, due to the axial compressive preload, and areeasily accommodated by the bond strengths of conventional polymerstructural adhesives. This approach allows one to use a rotor of verysimple geometry that is easy to forge and machine since it does not haveintegral shafts.

Referring to FIG. 21, the rotor 12 is a simple fixed or variablethickness disk without shafts as in FIG. 20. In this case, the shafts120 are welded to the rotor. In some embodiments, the shafts may bewelded to the motor with conventional fusion fillet welds betweencontact surface 126 and rotor 12. Following the welding operation,conventional heat treatment procedures remove stress concentrationsintroduced into the rotor at the weld locations. Since the rotor islifted directly by the magnetic off-loader, the stresses in the weldsare low.

In another embodiment, the shafts 120 are friction-welded to the rotorusing a high axial force 128 to press the shaft onto a rotating rotorblank. The contact surface 126 reaches a high temperature sufficient toweld the interface. Following the welding operation, conventional heattreatment procedures remove stress concentrations introduced into therotor at the weld. Since the rotor is lifted directly by the magneticoff-loader, the stresses in the welds are low.

Referring to FIG. 22, the rotor is constructed from several laminatedplates 130 that are adhesively bonded together using conventionalstructural adhesives. The only stress in the joints between thelaminations is gravity loading, which occurs when the rotor is lifted.This stress is low and easily accommodated by the adhesive tensilestrength. For example, for ten laminations, each 25 mm in thickness (1inch), the tensile stress in the first lamination joint (the most highlyloaded bonded joint) is less than 0.021 MPa (3 psi). Structuraladhesives have tensile strengths readily exceeding 7 MPa (1000 psi).Thin laminas can be individually heat-treated to higher strengths,thereby increasing the rotor energy density. In addition, laminatedrotors have a high degree of redundancy since flaw propagation in onelamina tends to be restricted by the adjacent laminas. In addition,failure of one lamina does not result in failure of the entire rotor.Also, since the laminas are thin, they are in a state of biaxial planestress when the rotor is spinning, which is a more uniform stress statethan the biaxial plane strain state that exists in a thick monolithicrotor. In addition, thin plates can be heat-treated to a higher yieldstrength than thick plates; thus, a rotor comprising of thin plateslaminated together will exhibit a higher energy density than in amonolithic rotor of the same total thickness.

Referring to FIG. 23, a composite fiber-reinforced ring is manufacturedusing a high-speed rotating cylindrical mold 132 into which is fed afiber bundle from a rotating spool 134 located inside the mold whosespin axis is parallel to the rotating mold axis. As the fiber bundle isunwound from the spool, it is held against the inside surface of therotating mold by centrifugal force. Room temperature curingpre-catalyzed thermosetting resin is sprayed from a nozzle 136perpendicular to the vertical wall of the rotating mold onto the fiberbundle lying against the wall. The high g-force provides adequatepressure for the liquid resin to infiltrate the fiber bundle as curingof the resin proceeds. When the cure is complete, the mold is removedand the ring ejected from the mold. This process is 10 to 50 timesfaster than filament winding, the conventional process for manufacturingcomposite rings. For example, fiber dispensing rates of 4500 m/min arepossible compared to filament winding rates of 60-90 m/min.Alternatively, a resin system that cures at elevated temperature may beused together with a method for heating the mold surface, either byinternal electrical resistance heaters, gas-fired heaters, or infraredlamps illuminating the mold from the inside. Alternatively, the rotatingmold has a central shaft and shaft lip seals so that infiltration andcuring may be done in vacuum to minimize voids in the composite.Additional spools may be simultaneously deployed such that processingtimes can be further reduced and/or different fibers (glass, carbon,KEVLAR®, metal wires, etc.) can be dispensed simultaneously or insequence, such that the final composite ring has a layered structure ofdifferent fiber types that may be advantageous in certain applications.Alternatively, different resin systems can be applied in sequence tovary properties radially. For example, a composite ring can be readilyfabricated in this manner with carbon fibers at its outside diameter andglass fibers at its inside diameter. Due to the high g-loading in thisapplication, void-free composite rings can be produced at high rates.

Referring to FIG. 24, a pre-impregnated and partially cured fiber bundle(tow preg, 138) is dispensed from a spool 134 as in FIG. 23 into ahigh-speed rotating cylindrical mold 132. An internal 142 (or external)heater heats the dispensed tow preg enabling it to flow and cure.

What is claimed is:
 1. A method of pre-conditioning a flywheel rotor,the method comprising: spinning a rotor at a rotational speed thatcauses a strain on the rotor to exceed a yield strength of a material ofthe rotor; and responsive to reaching a predefined amount of yielding,slowing the rotor back to a stationary position, wherein the spinningand slowing of the rotor causes an increase in the yield strength of thematerial of the rotor; and respinning the rotor to the rotational speed,wherein the respinning the rotor to the rotational speed causes a strainon the rotor that does not exceed an increased yield strength of thematerial of the rotor resulting from the increase in the yield strengthof the material of the rotor.
 2. The method of claim 1, wherein,respinning the rotor to the rotational speed causes a stress on thematerial of the rotor that does not exceed the increased yield strengthof the material of the rotor.
 3. The method of claim 1, furthercomprising monitoring a stress state of the rotor to determine when theyield strength of the material of the rotor has been exceeded.
 4. Themethod of claim 3, wherein monitoring the stress state of the rotorcomprises coating an outer surface of the rotor with a brittle paint andmonitoring the brittle paint for cracks.
 5. The method of claim 4,wherein spinning the rotor at a rotational speed that causes a strain onthe rotor to exceed a yield strength of a material of the rotorcomprises producing a crack pattern in the brittle paint.
 6. The methodof claim 4, wherein monitoring the brittle paint for cracks comprises:applying a strobe light to the rotor, wherein the strobe light issynchronized with the rotational speed of the rotor; and recording aprogression of the cracks in the brittle paint with a video camera. 7.The method of claim 6, wherein recording a progression of the crackswith a video camera comprises recording a progression of the cracks as afunction of the rotational speed.
 8. The method of claim 3, whereinmonitoring a stress state of the rotor to determine when the yieldstrength of the material of the rotor has been exceeded comprises:coupling strain gages to an outer surface of the rotor; and monitoringstrain distribution measurements of the strain gages as a function ofthe rotational speed.
 9. The method of claim 1, wherein spinning a rotorat a rotational speed that causes a strain on the rotor to exceed ayield strength of a material of the rotor and responsive to reaching apredefined amount of yielding, slowing the rotor back to a stationaryposition comprises increasing an energy storage density of the rotor.10. The method of claim 1, wherein spinning a rotor at a rotationalspeed that causes a strain on the rotor to exceed a yield strength of amaterial of the rotor comprises forming a zone of plastic deformationextending from a center of the rotor toward a periphery of the rotor.11. The method of claim 10, wherein forming a zone of plasticdeformation extending from a center of the rotor toward a periphery ofthe rotor comprises forming a zone of plastic deformation having aradius equal to about 1/√2 of a radius of the rotor.
 12. The method ofclaim 1, wherein spinning a rotor at a rotational speed that causes astrain on the rotor to exceed a yield strength of a material of therotor comprises forming a zone of compressive stress adjacent a centerof the rotor and a zone of tensile stress adjacent a periphery of therotor.
 13. The method of claim 1, further comprising selecting a rotordisc of the rotor to have a uniform thickness.
 14. The method of claim1, further comprising selecting the rotor to comprise a steel alloy. 15.A method of pre-conditioning a flywheel rotor, the method comprising:forming a zone of compressive stresses adjacent to a center of adisc-shaped rotor; and forming a zone of tensile stresses adjacent to aperiphery of the disc-shaped rotor.
 16. The method of claim 15, whereinforming a zone of compressive stresses and forming a zone of tensilestresses comprises imposing a radial temperature gradient using heaterson the disc-shaped rotor such that a higher temperature is appliedadjacent the center of the disc-shaped rotor than adjacent to theperiphery of the disc-shaped rotor.
 17. The method of claim 15, whereinforming a zone of compressive stresses and forming a zone of tensilestresses comprises: spinning a rotor at a rotational speed that causes astrain on the rotor to exceed a yield strength of a material of therotor; and responsive to reaching a predefined amount of yielding,slowing the rotor back to a stationary position, wherein the spinningand slowing of the rotor causes an increase in the yield strength of thematerial of the rotor.
 18. The method of claim 17, wherein spinning arotor at a rotational speed that causes a strain on the rotor to exceeda yield strength of a material of the rotor comprises forming a zone ofplastic deformation extending from the center of the rotor toward theperiphery of the rotor.
 19. The method of claim 18, wherein forming azone of plastic deformation extending from a center of the rotor towarda periphery of the rotor comprises forming a zone of plastic deformationhaving a radius equal to about 1/√2 of a radius of the rotor.
 20. Themethod of claim 17, further comprising respinning the rotor to therotational speed, wherein the respinning the rotor to the rotationalspeed causes a strain on the rotor that does not exceed an increasedyield strength of the material of the rotor resulting from the increasein the yield strength of the material of the rotor.